1. Field of the Invention
The invention relates to a method and a plant for the production of a metal strip, preferably a steel strip, in particular consisting of stainless steel and carbon steel, with a casting thickness of 1.0 mm to max. 20 mm, preferably 1.5 mm to max. 12 mm, and with excellent surface quality, using the two-roll casting method and further treatment stages.
2. Description of the Related Art
EP-A 776 984 already discloses a plant of this type for the production of a metal strip according to the two-roll casting method. This two-roll casting device is followed by a hot-rolling stand, by means of which the cast strip is rolled to form an intermediate product of predetermined strip thickness. To ensure a uniform delivery of the cast metal strip to the rolling stand, the latter is preceded by a driving-roller stand. A substantial disadvantage of this plant arrangement is that the casting speed in the two-roll casting device and the rolling speed in the rolling stand have to be constantly co-ordinated with one another and even minor speed deviations in one of the plant components give rise to reactions on other plant components which are detrimental to the quality of the product produced. Identical problems with the synchronization of the casting speed and rolling speed also arise in a casting-roll plant, such as is described in EP-A 760 397 and illustrated in FIG. 3. The strip cast in a two-roll casting plant is conveyed by a driving-roller stand and, before it enters the rolling stand, is held under tension by a compensating roller.
It is already known from JP-A63-48350 to cast metal strips consisting of permalloy and aluminium with a thickness of up to 1.0 mm according to the two-roll casting method, to store briefly the metal strip in an intermediate store, in which the metal strip is tautly tensioned by a compensating roller, or, according to other embodiments, in an intermediate store formed by a loop pit which the metal strip runs through, hanging freely, and subsequently to deliver the said metal strip to a strip-winding device. As a result of the brief intermediate storage, the two-roll casting device is separated functionally from the winding plant to the extent such that jolt-like movements in the metal strip which emanate from the strip winder do not react into the region of the casting plant and the high-temperature zone of the metal strip and lead to damage there. By virtue of the brief intermediate storage, there is also no need for a synchronization of the casting speed and winding speed. Due to the long metal-strip loop which fluctuates in length and which extends, hanging down freely under its own weight, directly from the casting gap and, by being deflected, undergoes an undefined pendulum movement, sharply fluctuating tensile stresses for the metal strip arise, which lead to the formation of cracks and to damage to the strip surface. Where relatively large strip thicknesses are concerned, the risk of cracking rises in the immediate vicinity of the casting gap owing to the increasing dead weight. Even when the metal strip forms a strip loop in a loop pit only after being supported by some supporting rollers, adverse reactions of the loop movement on the stress conditions in the metal strip in the region near the casting gap occur. The same difficulties also arise when plants, such as are described in EP-B 540 610 (WO-A 92/01524), EP-A 726 122 or WO-A 95/13156, are used to produce a metal strip. In all instances, a strip loop sagging freely under its own weight is formed immediately downstream of the two-roll casting device.
It is known, furthermore, from JP-A 63-238 963, in a casting plant, the mould of which is formed by rotating bands, to cast a metal strip in a thickness range of 15 to 50 mm. The metal strip is conveyed further on, at a regulated speed, by a pair of driving rollers and is guided through a loop pit prior to thickness reduction in a multi-stand hot-rolling mill. The strip sag of variable length in the loop pit causes different strip-tension conditions upon entry into the hot-rolling mill, with the result that adherence to a uniform strip quality is not ensured. In addition, the strip runs out of true laterally in the rolling stand.